1. Field of the Invention
The present invention relates to a protective device provided with a fuse element on a substrate.
2. Description of the Related Art
Protective devices obtained by forming fuse elements on a substrate are known as chip-type electric fuses. In addition, protective devices in which heating elements are placed in the vicinity of fuse elements are used in overvoltage prevention devices (Japanese Patent No. 2790433, Japanese Patent Application Laid-Open No. 8-161990).
FIGS. 2B and 2C are a plane view and a cross section, respectively, of such a protective device 1o. In the protective device 1o, electrodes 3a, 3b, and 3c are formed on a substrate 2, and solder foil spanning the electrodes 3a, 3b, and 3c is provided as a fuse element 4. The bottom surface of the electrode 3b is provided with a heating element 7 with an insulating layer 8 sandwiched therebetween. Heating current is supplied to the heating element 7 from an electrode 3o through conductors 3x and 3y. 
During the fabrication of the protective device 1o, the conductors 3x and 3y, heating element 7, insulating layer 8, and electrodes 3a, 3b, and 3c are formed on the substrate 2; solder paste 5 is then applied to the electrodes 3a, 3b, and 3c in the manner shown in FIG. 2A; a fuse element 4 is mounted on top; and a cover (not shown) is provided as needed. The protective device 1o thus obtained is commonly heated and mounted on a base circuit board by reflowing, soldering, or a similar technique.
To prevent the fuse element 4 from melting during the mounting of the protective device 1o, materials whose solidus point is higher than the mounting temperature of the fuse element 4 (in particular, whose solidus point is higher than the maximum temperature achieved during mounting, and whose liquidus point is even higher) are used for the fuse element 4. In addition, materials whose liquidus points are at or above the temperature maintained during heating and mounting are selected for the solder paste 5. When, for example, the temperature at which the protective device 1o is mounted on a circuit board is set to 250xc2x0 C., a material whose liquidus point is 250xc2x0 C. or higher is used for the solder paste 5 of the protective device 1o, and a material whose solidus point is higher than the liquidus point of the solder paste 5 is used for the fuse element 4.
When the above-described protective device 1o is used as a protective device in an apparatus to be protected, a malfunction in the apparatus causes an overcurrent to flow through the fuse element 4 and to melt it, or causes the heating current from the electrode 3o to be supplied to the heating element 7 through the conductors 3x and 3y, and the fuse element 4 to be melted by the generated heat. A drawback, however, is that the high liquidus point of the fuse element 4 increases the time between the beginning of the increase in the temperature of the fuse element 4 and the melting of the element, resulting in a protective device with an extended operating response. Increasing the time that elapses before the fuse element 4 is melted is disadvantageous in that the area where the protective device 1o is mounted on the base circuit board melts before the fuse element 4 does, causing the protective device 1o to separate from the base circuit board and inducing undesirable changes in peripheral mounted components, substrate wiring, and the like.
Another drawback is that the existence of the above-described relation between the liquidus points and solidus points of the fuse element 4 and solder paste 5 on the protective device 1o reduces the number of available materials and increases costs. In particular, attempts to bring the solidus point of the fuse element 4 closer to (or slightly above) the mounting temperature of the protective device 1o in order to improve the operating response are hampered by the scarcity of suitable fuse element materials having such solidus points.
An object of the present invention, which is aimed at overcoming the above-described drawbacks of prior art, is to improve the operating response, to widen the selection of materials for fuse elements, and to reduce manufacturing costs in a protective device obtained by forming a fuse element on a substrate.
The inventors perfected the present invention upon discovering that raising the liquidus point of the fuse element material in order to prevent the fuse element from melting during the mounting of a protective device obtained by forming the fuse element on a substrate dispenses with the need to increase the solidus point above the mounting temperature as long as the liquidus point is higher than the mounting temperature of the protective device; that is, keeping the solidus point below the mounting temperature, and the liquidus point above the mounting temperature allows the fuse element to perform its function because the fuse element is prevented from melting during mounting while being allowed to change its initial shape to a certain extent.
Specifically, the present invention provides a protective device in which a fuse element is provided to electrodes on a substrate, wherein the fuse element has a liquidus point higher than the mounting temperature of the protective device, and has a solidus point lower than the mounting temperature of the protective device.
These and other objects, features and advantages of the present invention are described in or will become apparent from the following detailed description of the invention.